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find_context_parameter() fails to detect Context parameter in callable class instances #1974
Description
Initial Checks
- I confirm that I'm using the latest version of MCP Python SDK
- I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue
Description
When registering a callable class instance as an MCP tool via FastMCP.add_tool(), the ctx: Context parameter is incorrectly exposed as an externally visible tool parameter instead of being injected by the framework. This is because find_context_parameter() uses typing.get_type_hints() which doesn't introspect the call method of callable class instances.
Suggested Fix
Update find_context_parameter() to handle callable class instances:
def find_context_parameter(fn: Callable[..., Any]) -> str | None:
from mcp.server.fastmcp.server import Context
# Handle callable class instances by using __call__ method
target = fn
if not (inspect.isfunction(fn) or inspect.ismethod(fn)):
if callable(fn) and hasattr(fn, "__call__"):
target = fn.__call__
try:
hints = typing.get_type_hints(target)
except Exception:
return None
# ... rest of function unchanged
Example Code
from typing import Annotated, Any
from mcp.server.fastmcp import Context, FastMCP
from mcp.server.session import ServerSession
from starlette.requests import Request
import pydantic
class MyTool:
def __init__(self, name: str):
self.__name__ = name
async def __call__(
self,
query: Annotated[str, pydantic.Field(description="Search query")],
ctx: Context[ServerSession, Any, Request],
) -> str:
"""Performs a search."""
return f"Results for: {query}"
mcp = FastMCP(name="Test Server")
mcp.add_tool(MyTool(name="my_search"), name="my_search", description="Search tool")
# When listing tools, 'ctx' appears as a required parameter in the schemaPython & MCP Python SDK
1.26.0
Activity
maxisbey commentedon Feb 10, 2026
This is part of the broader context handling refactor planned for V2 — see #1684 for tracking.
AI Disclaimer
fix: handle callable class instances in find_context_parameter()